1. Field of the Invention
The present invention relates to a semiconductor device such as a flip chip, a BGA (Ball Glid Array), an CSP (Chip Scale Package), or the like, an electrode of the semiconductor device, and a method of manufacturing the electrode.
2. Description of the Related Art
A flip chip, a BGA, an CSP and so on are conventionally known as semiconductor devices which can satisfy a demand to increase the number of pins of the device. Such a semiconductor device has, for example, an electrode structure called a "solder bump structure" manufactured by electroless plating (for example, see JP-A-8-306816, and JP-A-1-185920).
FIG. 8 is a sectional view illustrating an electrode structure of a flip chip which is an example of a conventional electrode structure. This electrode has a pad 50 formed of metal such as aluminum (Al) or the like. The pad 50 is formed on a chip substrate 51, and on the surface thereof, with an electroless barrier metal film 52 for protecting the pad 50.
The electroless barrier metal film 52 has a double-layer structure constituted by an electroless diffusion prevention film 52a for preventing Al which is a material of the pad 50 from diffusing, and an electroless oxidation prevention film 52b for preventing the surface of the pad 50 and the electroless diffusion prevention film 52a from being oxidized. For example, an Ni--P film is used for the electroless diffusion prevention film 52a. On the other hand, for example, an electroless Au film is used as the electroless oxidation prevention film 52b.
The flip chip is mounted onto a circuit board in such a manner that a solder bump 53 is attached to the surface of the electroless barrier metal film 52, and then the solder bump 53 is melted on the wiring pattern. The reference numeral 54 represents a passivation film for preventing the pad 50 from being oxidized.
However, in the electrode with the above-mentioned solder bump 53 attached thereto, a columnar intermetallic compound layer 60 was formed in the solder bump 53 so as to erect on the surface of the electroless barrier metal film 52 as shown in FIG. 9A. Therefore, there was a problem that the solder bump 53 was apt to defectively break when force F was applied to the solder bump 53 from the outside, as shown in FIG. 9B.
More specifically, the electroless Au film 52b melts into the solder bump 53 when the solder bump 53 is attached to the electroless barrier metal film 52. As a result, the solder bump 53 comes into contact with the surface of the electroless Ni--P film 52a. On the other hand, generally, the electroless Ni--P film 52a is microcrystalline or amorphous because of eutectoid of reducer elements P in a reducer used for electroless plating. As a result, Ni in the surface of the electroless Ni--P film 52a enters the solder bump 53. Ni entering the solder bump 53 performs a chemical reaction with Sn in the solder bump 53. Consequently, an intermetallic compound layer 60 of Ni.sub.3 Sn.sub.4 is produced in the solder bump 53.
On the other hand, when Ni is released from the electroless Ni--P film 52a, the content of the reducer elements P in the electroless Ni--P film 52a increases in the vicinity of the surface thereof (for example, increases from 9.5 wt % to 17 wt %). As a result, an amorphous layer containing the reducer elements P at high concentration (P-rich amorphous layer) is formed in the electroless Ni--P film 52a in the vicinity of the surface thereof. Because of existence of this P-rich amorphous layer, the growth of the above-mentioned intermetallic compound layer 60 is blocked in a direction h along the surface of the electroless Ni--P film 52a. Consequently, the produced intermetallic compound layer 60 grows in its height-wise direction v.
In such a manner, the intermetallic compound layer 60 grows in its height-wise direction v. Therefore, for example as shown in FIG. 9A, the intermetallic compound layer 60 is formed in the form of a column in the solder bump 53. In this case, the bonding area between the intermetallic compound layer 60 and the electroless Ni--P film 52a is very small, and the height of the intermetallic compound layer 60 is comparatively high, for example, 1 (.mu.m) or more. Further, an P-rich amorphous layer is formed in the electroless Ni--P film 52a in the vicinity of the surface thereof.
Therefore, a bonding force between the solder bump 53 and the electroless Ni--P film 52a is very small. Accordingly, when a force F is applied to the solder bump 53 from the outside during the work of soldering by melting the solder bump 53, the intermetallic compound layer 60 itself is broken, or defective breaking is produced between the intermetallic compound layer 60 and the P-rich amorphous layer as shown in FIG. 9B. Therefore, an electrode cannot be attached to the wiring pattern. After all, the device often becomes a rejected product.